In engine systems for internal combustion engines, exhaust gas is recirculated in order to reduce the nitrogen oxide component of the exhaust gas. Due to the increased soot component in the exhaust gas brought about by the exhaust gas recirculation, the quantity of recirculated exhaust gas (indicated by the exhaust gas recirculation rate: EGR rate) is limited with the aid of a constant exhaust gas recirculation rate as a compromise between the nitrogen oxide component and the soot component in the exhaust gas.
The recirculated exhaust gas is normally conducted through a cooler (EGR cooler) for cooling. Cooling the recirculated exhaust gas allows for a higher EGR rate at a constant intake manifold pressure and may thus have a significant influence on minimizing raw emissions.
Modern control arrangements such as, e.g., the model-based charge control (MCC) make it possible to control the EGR rate and have the advantage over conventional air mass controllers of being able to keep the emissions of the internal combustion engine in narrower tolerances. The required control variable of the EGR rate is generally calculated with the aid of an air system model which assumes the model of an intact, ideal cooler for modeling the temperature of the cooled exhaust gas.
The efficiency of the EGR cooler, however, may change while the internal combustion engine is in operation such that the cooling performance varies. Due to the changed density of the recirculated exhaust gas, the variation of the cooling performance results in a change of the EGR rate and may thus result in a significant fluctuation of the emission of the internal combustion engine.
Furthermore, because the cooler for cooling the recirculated exhaust gas (EGR cooler) is relevant in terms of emissions, the law requires that the cooling function be monitored in connection with the on-board diagnosis.